The invention relates to a system for searching idle switching time slots defining a selected four-wire communication connection through a switching network working on the basis of time and space-division. The network is of the type wherein switching operations in time domains are carried out by time switches at the input and output of the network, and switching operations in the space domain are carried out in between these time switches. This network comprises a number of address registers each suitable for address-controlled reading out of the time switches associated to the address registers; and a control arrangement required for controlling and coordinating the necessary switching operations. This control arrangement is designed for searching idle switching time slots which can be utilized for a requested four-wire communication connection by means of a recording memory. The recording memory records for each time switch the idle/busy condition of switching time slots, under the control of which the time switches associated to the address registers are read out. A switching time slot is to be regarded as a time interval during which a relevant time switch is read out at a specific address position and the thus read speech sample is conveyed from the transmit end to a receive end of a relevant four-wire communication.
Generally, each main group comprising several groups of time channels is associated with an input time switch for traffic due to enter the switching network, and an output time switch for traffic due to leave the switching network. Thereto the switching network is of such a configuration and connected to the groups of time channels such that a four-wire communication connection can be formed between time channels belonging to one and the same group as well as between time channels belonging to different groups. Such a four-wire communication connection is composed of a pair of two-wire connections which are generally established on two different, correlated switching time slots. If traffic load is offered to a switching network of the type described hereinbefore, more specifically, if this network has a correlated go and returnpath, and if no further measures are taken, the chance of blocking will be considerably higher if not unacceptably higher for requested internal group connections, i.e. communication connections between the channels of one and the same group, than the blocking chance for external group connections, i.e. communication connections between time channels of different groups. As a compromise, the total number of time channels operated by the switching network could be selected to be lower than the number of channels for which the switching network was originally designed. However, such a selection is unacceptable as in that case the available hardware will not be used to full capacity.